1. Field of the Invention
This invention relates generally to paper size used in the manufacture of paper. More particularly, this invention relates to size used in the manufacture of sized paper wherein the fibrous furnish used to make the paper has high hardness (i.e., an excess of calcium or magnesium ions). The invention further relates to improvements in the sized paper manufacturing process involving size application.
2. Description of Related Art (Including Information Disclosed Under 37 CFR 1.97 and 37 CFR 1.98)
The sizing of paper is an old and well-established art, and a wide variety of materials have been proposed heretofore for this purpose. Sizing agents are typically added to cellulose fibers to impart resistance of the paper to the penetration of liquids. Resistance to liquid penetration is necessary to prevent the paper from breaking down when the paper is exposed to water during the papermaking operation, e.g., passing through a size press starch solution prior to drying. Resistance to liquid is also necessary so that the print quality can be maintained upon application of water-based inks to the paper surface. As is well known, the sizing agent may be applied to the fibers during the papermaking operation, in which case the process is called internal sizing (also known as beater or engine sizing), or it may be applied to the surface of the paper after web formation, in which case it is called external or surface sizing. Internal sizing serves primarily to increase resistance to water penetration; whereas external sizing does that as well as improve other surface properties, such as strength and/or printability. Certain papers, such as those used in aseptic packaging, may be produced via a two step sizing process comprising an internal size step and a surface size step, as taught in U.S. Pat. No. 5,308,441 to Kern.
The sizing agents used in the paper industry cover a wide range of materials, including rosin size (such as aluminum salts of abietic acid), synthetic resins, glue or gelatin (animal-derived) starch or cellulose derivatives, lattices, polyvinyl alcohol, wax emulsions, and many other similar materials. Some of these, such as rosin size, are used exclusively as internal sizing agents, while others, such as polyvinyl alcohol, are used almost entirely as surface sizing agents. Some materials, such as wax, may be applied by either internal or surface sizing procedures. Rosin size and synthetic resins are the two principal materials used for the internal sizing of paper. Other materials may be used only in small quantities. Nevertheless, rosin size remains, by far, the most important internal sizing agent for bleached board products, while for fine paper products, particularly free sheet, synthetic resins are preferred, primarily due to the use of precipitated calcium carbonate in such products.
The difficulties involved in the sizing of calcium carbonate filled papers with the use of common sizing ingredients, such as rosin and alum, have been long recognized.
U.S. Pat. No. 2,114,509 teaches a method for producing sized paper including a filler containing a calcium salt and an acidic precipitant wherein the mixture of furnish and sizing components is maintained out of contact with the outside atmosphere. Under these conditions, the escape from the mixture of substantial quantities of carbon dioxide is prevented. The method is disclosed as being capable of achieving good sizing while substantially eliminating foam. The benefit of carbon dioxide as a sizing enhancer was recognized in U.S. Pat. No. 5,378,322, where the patentee dissolved CO2 in an aqueous vehicle to catalyze the reaction between the alkylketene dimer sizing agent and the cellulose in the papermaking fibers. Also, U.S. Pat. No. 2,195,600 discloses the use of a protective colloid for rosin in a papermaking furnish containing alkaline filler, to protect the rosin prior to being precipitated by alum. The present invention, as herein disclosed, provides a more effective sizing method for high hardness conditions, and is distinguished by the aforesaid prior art by producing a size material by pre-reacting alum and liquid rosin size before the mixture is introduced into the papermaking furnish.
In rosin or soap rosin sizing, papermaking fibers are rendered hydrophobic by precipitating the rosin with alum to produce aluminum rosinate (see article entitled xe2x80x9cChemistry of Rosin Sizing,xe2x80x9d by Edward Strazdins, TAPPI Journal, January 1981, vol. 64, No. 1, pp. 31-34). As Strazdins pointed out, the rosin-alum sizing system is useful for various practical reasons, including the following:
(a) the aluminum rosinate precipitate is an excellent retention aid,
(b) the degree of sizing is easy to control,
(c) the handling of rosin size is relatively simple,
(d) the problems of repulping waste are minimal,
(e) the size is compatible with other wet-end additives, and
(f) the costs of sizing with rosin are relatively low.
A significant drawback to the use of rosin or soap rosin size, however, is the reduction of paper strength due to the strong attachment of the aluminum rosinate precipitate to papermaking fibers. Because of this, the size precipitate is not capable of relocating during the web consolidation and interferes with fiber bonding. The efficiency of alum-rosin sizing suffers even more if the papermaking furnish has a high hardness, or contains large amounts of calcium or magnesium ions. The presence of calcium, for example, produces calcium rosinate, which is an inferior sizing agent to aluminum rosinate and, thus, reduces sizing efficiency.
A common method employed for offsetting the negative effect of high hardness on sizing is to use reverse sizing. With this technique, the alum is added to the furnish before the rosin size. Thus, reverse sizing represents an improvement over conventional rosin sizing of high hardness furnish, since the alum is already in the furnish when the rosin size is added. While reverse sizing can lead to some sizing improvement in a high hardness furnish, the alum must still compete with any multivalent cations in the furnish. Accordingly, the present invention is directed to a rosin size material that achieves even further improvement in the sizing of high hardness furnishes, using either standard or reverse sizing methods, than can be achieved even with reverse sizing with conventional rosin size.
An object of the invention of pending application Ser. No. 09/329,842 was to provide an alum-rosin size composition to achieve maximum effectiveness even under high hardness conditions. A further object of said invention, and a benefit to use of the novel invention size material, was to improve the efficiency of alum-rosin sizing systems in high hardness furnishes in order to optimize the amount of size needed to achieve the desired level of sizing. The efficiency of the composition was shown by examples of its addition to the thick stock (xcx9c4% consistency) at the normal thick stock addition point, being the machine chest, at pH 4.8, in lieu of the conventional addition of soap size to the machine chestxe2x80x94it being well-known that sizing efficiency is reduced as the pH increases up to xcx9cpH 6.5.
The time of application of the novel composition in thick stock was typical of normal papermaking systems at about 20 minutes prior to headbox discharge. This is conventional practice for soap size as well. The efficiency of the manufacture of the small particle aluminum rosinate size, formed external to the papermaking stock system was taught in the ""842 application to be in the avoidance of competing reactions and chemical interferences, allowing the alum to remain free to react with the rosin size.
The present invention improvement of such technology resides in the ability to add the aluminum rosinate size composition unconventionally in thin stock, where retention times are on the order of less that 1 minute. The novel aluminum rosinate size composition is highly charged and is able to be quickly adsorbed onto the fiber surfaces to permit efficient sizing in thin stock. The quick sizing avoids concern with rising pH over time and increases sizing efficiency.
The improved sizing agent, produced by pre-reacting the mixture into the papermaking furnish, wherein the alum is free to react with the rosin size without any competition from other cations, provides the highly charged nature of the sizing composition permitting efficient sizing via thin stock addition.